The present invention relates to a liquid crystal device for use in a liquid crystal display device or an optical shutter array, etc., and more particularly, to a liquid crystal device having improved display and driving characteristics, derived from improved initial alignment or orientation of liquid crystal molecules.
Hitherto, there has been known a liquid crystal device using a TN (twisted nematic) type liquid crystal, as shown in "Voltage-Dependent Optical Activity of a Twisted Nematic Liquid Crystal" by M. Schadt and W. Helfrich, Applied Physics Letters Vol. 18, No. 4 (Feb. 15, 1971), pp. 127-128. However, such a TN-liquid crystal device involves a problem of causing crosstalk when it is constructed with a high density of picture elements formed with a matrix electrode structure driven in a time-division manner, so that the number of picture elements has been restricted.
Further, there has been also known a display device of a system wherein picture elements are respectively provided with a thin film transistor (TFT) and are switched thereby one at a time. This system however involves a problem in that it requires a complicated step of forming TFTs on a substrate, so that it is difficult to form a display device of a large area.
In order to obviate the above-mentioned drawbacks of the conventional types of liquid crystal devices, Clark and Lagerwall have proposed the use of a liquid crystal device having bistability (Japanese Laid-Open patent application No. 107216/1981, U.S. Pat. No. 4,367,924, etc.). As the bistable liquid crystal, a ferroelectric liquid crystal having chiral smectic C (SmC*) phase or H (SmH*) phase is generally used. The ferroelectric liquid crystal has bistability, i.e., has two stable states comprising a first stable state and a second stable state, with respect to an electric field applied thereto. Accordingly, in contrast to the conventional TN-type liquid crystal in the above-mentioned device, the liquid crystal is oriented to the first stable state in response to one electric field vector and to the second stable state in response to the other electric field vector. Further, this type of liquid crystal very quickly assumes either one of the above-mentioned two stable states in reply to an electric field applied thereto and retains the state in the absence of an electric field. By utilizing these properties, essential improvements can be attained with respect to the above-mentioned difficulties involved in the conventional TN-type liquid crystal device.
However, in the ferroelectric liquid crystal devices provided heretofore, a uniform orientation or alignment state of a liquid crystal cannot satisfactorily be formed, whereby the optical modulation device cannot actually perform adequately. For this reason, several methods have been proposed to provide a uniform orientation state of a ferroelectric liquid crystal showing bistability in the presence of a surface which has been subjected to a rubbing treatment or an oblique vapor deposition treatment. It is already known that a uniform alignment state of a bistable ferroelectric liquid crystal can be obtained by using a substrate subjected to the above mentioned rubbing treatment or oblique vapor deposition treatment.
However, according to our experiment, we have found that the thus obtained bistable state is not ideal, while as disclosed by Clark and Lagerwall in the above-described publications.
More specifically, according to Clark and Lagerwall, the tilt angle (angle .circle.h shown in FIG. 3 as will be explained hereinafter) in a chiral smectic phase of a nonspiral structure for realizing the bistability should be identical to a tilt angle (angle .circle.H corresponding to one half of the apex angle of a cone shown in FIG. 2 explained hereinafter) in the corresponding chiral smectic phase having a spiral structure. In fact, however, the tilt angle .circle.h in the nonspiral structure is smaller than the tilt angle .circle.H in the spiral structure. Further, it has been found that the difference between the tilt angle .circle.h in the nonspiral structure and the tilt angle .circle.H in the spiral structure is attributable to a twisted arrangement of liquid crystal molecules in the nonspiral structure. More specifically, as shown in FIG. 5, in a chiral smectic phase of a nonspiral structure, the liquid crystal molecules are continuously twisted so as to form a twist angle .delta. from the axis 52 of the liquid crystal molecules contacting the upper substrate to the axis 53 of the liquid crystal molecules contacting the lower substrate along a twisted arrangement direction 54 with respect to a normal to the substrates. This arrangement causes a phenomenon that the tilt angle .circle.h in the nonspiral structure is smaller than the tilt angle .circle.H in the spiral structure. Line 51 denotes a uniaxial orientation axis provided by a rubbing treatment or an oblique vapor deposition treatment applied to the upper and lower substrates.
In the case of a liquid crystal device utilizing the birefringence of a liquid crystal, a transmittance with right angle cross nicols is expressed by the following equation: EQU I/I.sub.0 =sin.sup.2 4 .circle.h .multidot.sin.sup.2 (.DELTA.nd/.lambda.).pi.,
wherein I.sub.0 denotes an incident light intensity, I a transmitted light intensity, .circle.h a tilt angle, .DELTA.n a refractive index anisotropy, d the thickness of a liquid crystal layer, and .lambda. the wavelength of an incident light. The above-described tilt angle .circle.h in the nonspiral structure appears as an angle between the average molecular axis directions of the liquid crystal molecules in the first and second orientation states respectively in a twisted arrangement. According to the above equation, the maximum transmittance is given when the tilt angle .circle.h is 22.5.degree.. However, the tilt angle .circle.h in a nonspiral structure realizing bistability is of the order of 10.degree., so that the transmittance is of the order of 3-5% which is not sufficient for application to a display device.
Further, in the ferroelectric liquid crystal device provided heretofore, there is involved a problem that stabilization energy levels for a first orientation state and a second orientation state are not the same with each other under application of electric field vectors. More specifically, the magnitude of the electric field required for changing the first orientation state to the second orientation state is different from the magnitude of the electric field required for reversing the second orientation state to the first orientation state. Further, there is also observed a phenomenon (returning phenomenon) that a second orientation state produced by changing from a second orientation state under application of an electric field vector is lost without showing a proper memory effect to be returned to the original first orientation state when the electric field is removed.
These two phenomena may be attributed to a condition that the first stable orientation state and the second stable orientation state under bistability condition are not perfectly symmetrical to each other, but rather, one stable orientation state is more stable than the other stable orientation state. Such states are herein referred to as "unsymmetric two stable states". Such unsymmetric two stable states have caused a difference in thresholds when switching is effected between two stable orientation states to provide a problem in driving.
Further, there also results in mixing of another domain based on the first orientation state in a picture element where a uniform domain of the second orientation state is to be formed, thereby to cause lowering in light transmittance or poor light interrupting state.